Looper, Crossflow, what are they?

[N3UP]

I see many references to loopers and crossflow motors on this forum.

Being new to working on outboards, and having only ever owned a Merc before, I'd like to know what those terms mean.

So far I am impresssed by the parts availability for the older OMC motors.
I have given up on my old Merc because of parts non-availability.

Educate me!

 

[metriccrescentwrench]

Re: Looper, Crossflow, what are they?

"Looper" and "crossflow" basically refers to the design of the combution chamber. Crossflows have pistons with a ridge to make the air/fuel mixture flow where it's supposed to, loopers don't.

Someone else chime in with more details.

And yes, I like the parts availability also. One of my engines is an '81 Johnson (which is "new" compared to many of the engines guys here have), I've never had any trouble getting parts at the dealership across the street from where I work.

 

[rolmops]

Re: Looper, Crossflow, what are they?

The most visible difference is the top of the pistons.In a looper the top is flat,while in a crossflow one side of the piston is higher then the other.
A 2 stroke has its exhaust stroke and intake stroke at the same time.While exhaust leaves the cylinder,fresh mix comes into the cylinder
A cross flow typically has one intake and one outlet port at the bottom of the cylinder.Because of the height difference of the piston top,the fresh fuel mix is directed upward toward the sparkplug while the exhaust gasses leave the cylinder.
This system is a bit wasteful because part of the fresh fuel will get sucked out along with the exhaust gas without ever having been ignited.
A looper is a product of the seventies. Instead of one intake,there are several in the cylinder wall.They are placed strategically so that the upward stroke will concentrate the fuel mix close to the spark center.this makes for better fuel use.On the down stroke there is still some fresh fuel loss,but the exhaust is built in a way that it sends a sound wave that causes a push back into the cylinder of the fuel mix that would otherwise be lost.
The flat piston head is just part of this looper design.
I am sure that others can explain it much better,but I hope that this helps.
Johnson evinrude started with loopers in 1969,while Mercury likes cross flow better.

Hit this website and see what a crossflow does:
http://www.animatedengines.com/twostroke.shtml

now see a looper.

http://en.wikipedia.org/wiki/File:Arbeitsweise_Zweitakt.gif

 

[oldgradywhite]

Re: Looper, Crossflow, what are they?

A crossflow has a high dome pistion and the intake and exhaust ports are across from each other.

A looper had a flat piston and the ports are not across from each other.

 

[F_R]

Re: Looper, Crossflow, what are they?

Ever hear a chainsaw scream at 10,000 RPM? They are loopers.

The fuel/air charge entering the cylinder on a crossflow runs smack up against a wall--the piston dome. Then it has to make a right-angle turn toward the top of the cylinder. Any time the charge has to change directions like that, it is a heavy price to pay in efficiency loss. The charge in a looper is aimed toward the top of the cylinder so it doesn't run into a wall like the cross-flows. That is why they are so much more efficient, which translates to more power from a smaller engine, and better fuel economy while it is doing it. And the pistons are much lighter without that heavy dome, which means less reciprocating weight--another energy waster in the crossflows. That low recriprocating weight is one reason a chainsaw can scream at 10,000 RPM under no-load conditions and not fly apart.

 

[N3UP]

Re: Looper, Crossflow, what are they?

Johnson evinrude started with loopers in 1969,while Mercury likes cross flow better.

Ok, I get the idea.
So the 2 Johnny 85's that I have here (a 77 and a 74) are loopers then?
I got the impression from some of the posts that the loopers were newer, and the 70's motors were crossflow.

Am I confused?

 

[Haffiman]

Re: Looper, Crossflow, what are they?

Ok, I get the idea.
So the 2 Johnny 85's that I have here (a 77 and a 74) are loopers then?
I got the impression from some of the posts that the loopers were newer, and the 70's motors were crossflow.

Am I confused?

Yes, you are, and you are probably not alone.
OMC kept the cross-flow on the bigger engines up to generally mid 80's, they even produced the 115/90 up to 95!
There are advantages with both systems and most people never agrees which one is the best. It may go down to which characteristics you want from your engine, not to forget production price which often is the ruling factor by the manufacturer!!!
Your 2 Johnny's are cross engines and of the 'family' that went on almost unchanged to 95!!!